Wednesday, February 29, 2012

Von Baer (Feb 29, 1792 - Nov 28, 1876) was a Prussian-Estonian embryologist who discovered the mammalian egg (1827) and the notochord. He established the new science of comparative embryology alongside comparative anatomy with the publication of two landmark volumes (in 1828 and 1837) covering the range of existing knowledge of the prebirth developments of vertebrates.

He showed that mammalian eggs were not the follicles of the ovary but microscopic particles inside the follicles. He described the development of the embryo from layers of tissue, which he called germ layers, and demonstrated similarities in the embryos of different species of vertebrates.

The earliest known horse, Sifrhippus sandae, first appeared in the forests of North America more than 50 million years ago. It weighed in at around 12 pounds--and it was destined to get much smaller over the ensuing millennia.

Sifrhippus lived during the Paleocene-Eocene Thermal Maximum, a 175,000-year interval of time some 56 million years ago in which average global temperatures rose by about 10 degrees Fahrenheit. About a third of mammal species responded with a significant reduction in size during this time, some by as much as one-half.

Sifrhippus shrank by about 30 percent, to the size of a small house cat--about 8.5 pounds--in the first 130,000 years, then rebounded to about 15 pounds in the final 45,000 years. Secord says that the finding raises important questions about how plants and animals will respond to rapid change in the not-too-distant future.

Ornithologists, Secord says, have already started to notice that there may be a decrease in body size among birds. link

Wednesday, February 22, 2012

Sir Charles Lyell attended Oxford University at age 19. Lyell's father was an active naturalist. Lyell had access to an elaborate library including subjects such as Geology.

When Lyell was at Oxford, his interests were mathematics, classics, law and geology. He attended a lecture by William Buckland that triggered his enthusiasm for geology. Lyell originally started his career as a lawyer, but later turned to geology. He became an author of The Geological Evidence of the Antiquity of Man in 1863 and Principles of Geology. Lyell argued in this book that, at the time, presently observable geological processes were adequate to explain geological history. He thought the action of the rain, sea, volcanoes and earthquakes explained the geological history of more ancient times.

Lyell rebelled against the prevailing theories of geology of the time. He thought the theories were biased, based on the interpretation of Genesis. He thought it would be more practical to exclude sudden geological catastrophes to vouch for fossil remains of extinct species and believed it was necessary to create a vast time scale for Earth's history. This concept was called Uniformitarianism. The second edition of Principles of Geology introduced new ideas regarding metamorphic rocks. It described rock changes due to high temperature in sedimentary rocks adjacent to igneous rocks. His third volume dealt with paleontology and stratigraphy. Lyell stressed that the antiquity of human species was far beyond the accepted theories of that time.

Charles Darwin became his dear friend and correspondent. Darwin is quoted saying, "The greatest merit of the Principles was that it altered the whole tone of one's mind, and therefore that, when seeing a thing never seen by Lyell, one yet saw it through his eyes."

Tuesday, February 21, 2012

Atmospheric oxygen really took off on our planet about 2.4 billion years ago during the Great Oxygenation Event. At this key juncture of our planet’s evolution, species had either to learn to cope with this poison (O2)that was produced by photosynthesizing cyanobacteria or they went extinct. It now seems strange to think that the gas that sustains much of modern life had such a distasteful beginning.

So how and when did the ability to produce oxygen by harnessing sunlight enter the eukaryotic domain, that includes humans, plants, and most recognizable, multicellular life forms? One of the fundamental steps in the evolution of our planet was the development of photosynthesis in eukaryotes through the process of endosymbiosis.

This crucial step forward occurred about 1.6 billion years ago when a single-celled protist captured and retained a formerly free-living cyanobacterium. This process, termed primary endosymbiosis, gave rise to the plastid, which is the specialized compartment where photosynthesis takes place in cells.

Basic understanding of much of the subsequent evolution of eukaryotes, including the rise of plants and animals, is emerging from the sequencing of the Cyanophora paradoxa genome, a function-rich species that retains much of the ancestral gene diversity shared by algae and plants. New research provides conclusive evidence that all plastids trace their origin to a single primary endosymbiosis. link

Wednesday, February 08, 2012

The Max Planck Institute for Evolutionary Anthropology, in Leipzig, Germany, has completed the genome sequence of a Denisovan, a representative of an Asian group of extinct humans related to Neandertals.

In 2010, Svante Pääbo and his colleagues presented a draft version of the genome from a small fragment of a human finger bone discovered in Denisova Cave in southern Siberia. The DNA sequences showed that this individual came from a previously unknown group of extinct humans that have become known as Denisovans. Together with their sister group the Neandertals, Denisovans are the closest extinct relatives of currently living humans.

The Leipzig team has now developed sensitive novel techniques which have allowed them to sequence every position in the Denisovan genome about 30 times over, using DNA extracted from less than 10 milligrams of the finger bone. The now-completed version of the genome allows even the small differences between the copies of genes that this individual inherited from its mother and father to be distinguished. This Wednesday the Leipzig group makes the entire Denisovan genome sequence available for the scientific community over the internet.

The genome represents the first high-coverage, complete genome sequence of an archaic human group - a leap in the study of extinct forms of humans. “We hope that biologists will be able to use this genome to discover genetic changes that were important for the development of modern human culture and technology, and enabled modern humans to leave Africa and rapidly spread around the world, starting around 100,000 years ago” says Pääbo. The genome is also expected to reveal new aspects of the history of Denisovans and Neandertals.

The group plans to present a paper describing the genome later this year. “But we want to make it freely available to everybody already now” says Pääbo. “We believe that many scientists will find it useful in their research”.

Friday, February 03, 2012

Mantell (Feb. 3, 1790 – Nov. 10, 1852), a physician of Lewes in Sussex in southern England, had for years been collecting fossils in the sandstone of Tilgate forest, and he had discovered bones belonging to three extinct species: a giant crocodile, a plesiosaur, and Buckland's Megalosaurus. But in 1822 he found several teeth that "possessed characters so remarkable" that they had to have come from a fourth and distinct species of Saurian. After consulting numerous experts, Mantell finally recognized that the teeth bore an uncanny resemblance to the teeth of the living iguana, except that they were twenty times larger. In this paper, the second published description of a dinosaur, he concluded that he had found the teeth of a giant lizard, which he named Iguanodon, or "Iguana-tooth."

Mantell illustrated his announcement with a single lithographed plate. Mantell included at the bottom of the plate a drawing of a recent iguana jaw, which is shown four times natural size, and for further comparison, he added views of the inner and outer surface of a single iguana tooth, "greatly magnified."

The traditional story that Mantell's wife found the first teeth in 1822, while the doctor was visiting a patient, appears, alas, to be unfounded.

Any student of biology, or anyone with an interest in the natural world, will be familiar with Ernst Mayr who passed away on February 3rd, 2005 in Bedford, Mass. Born in Kempton, Germany he joined the American Museum of Natural History as a curator in 1931. In 1953 he left the museum to work at Harvard University where he stayed until his retirement in 1975.

While working on the problem of speciation in the birds of New Guinea, Mayr realized that the multitude of species and and subspecies that he saw could best be explained as being a snapshot of evolution in action. He suggested that new species could arise when the range of one species was fractured long enough for members in different parts of the range to evolve characters that would not allow individuals to reproduce when they were brought back together again. This lead to him developing the “biological species concept” in which species are defined as populations of interbreeding organisms rather than just a collection of characters. This idea, along with his theory of “allopatric speciation” was published in his book “Systematics and the Origin of the Species” (1942) and later contributed to the “Punctuated Equilibrium” theory of Niles Eldredge and Stephen Jay Gould.

Ernst Mayr was himself inspired by the work of geneticist Theodosius Dobzhansky on the fruit fly Drosophila melanogaster and his book “Genetics and the Origin of the Species” (1937). These two men, together with the paleontologist George Gaylord Simpson, combined the sciences of genetics, zoology and paleontology into what is now known as “the new synthesis” that provides the modern experimental underpinning to the concepts that Charles Darwin presented in his book, “On the Origin of the Species” .

For anyone interested in learning more about modern evolutionary theory I’d recommend Mayr’s recent book “What Evolution Is” (2002). It’s written in an engaging and readable format from the perspective of someone who’s thought about evolution all his life.